the present assessment are insufficient to initialize such a model because there is no available directional spectral information.
Validity of a Limited Dataset
An additional minor concern in the theoretical wave assessment is the limited statistical inference from the 51-month dataset. Although NREL conducted a “typicality” study to demonstrate the adequacy of the dataset, one could still argue whether the results of short time series are valid. A simpler approach could be to use confidence intervals to reflect the accuracy of the assessment. For example, when using a 20-year time series, the significant wave height representing a 50-year event on the East Coast is on the order of 8.5 m, with a 95 percent confidence interval between 7.5 and 9.5 m. This represents an expected theoretical power varying by a factor of 1.6 between the limits of the confidence interval; the mean value is accurate in a confidence interval of ±25 percent. Using a 51-month time series instead of 20 years significantly increases the range of the 95 percent confidence interval, although it could still be quantified.
Similarly, the occurrence of extreme events is not captured well in the 51-month time series, as acknowledged by the NREL validation group.5 As a result, the cumulative probability distribution curves might be less accurate for large waves. It is unclear how this affects the results, but an accurate evaluation of the confidence interval for extreme events will be needed to assess device survivability.
Scalar Power Density
A further concern related to the theoretical resource assessment is the use of the unit-circle approach. This approach has the potential to double-count a portion of the wave energy if the direction of the wave energy flux is not perpendicular to the line of interest or if there is significant wave reflection from the shore. This technique was the subject of criticism in the committee’s interim report (Appendix B). The assessment group responded to this point, and its final report correctly computes the wave-energy flux across lines parallel to the coast by integrating only the component of the wave energy flux vector that crosses the line (i.e., the normal component). It retains the results from the previous unit-circle approach in their report and shows that the line integral is 56-87 percent of the unit-circle estimate, depending on location (see Tables 2.16-2.19 in EPRI, 2011). Despite acknowledging the bias of the unit-circle approach for estimating
5 G. Scott, National Renewable Energy Laboratory. “Validation and display of wave energy resource estimates,” Presentation to the committee on February 8, 2010.